Intro to EKGs Flashcards
electrical conduction pathway through the heart
SA (sinus) node → atria → AV node → bundle of His → right and left bundle branches → Purkinje fibers → ventricles
note - left bundle branch divides into left anterior fascicle and left posterior fascicle
SA node (sinus node) - general
*located in upper part of crista terminalis near SVC opening
*“pacemaker” inherent dominance with slow phase of upstroke
*has the fastest automaticity of all the electrically active regions of the heart
AV node - general
*located in interatrial septum near coronary sinus opening
*blood supply usually from the RCA
*100msec delay allows time for ventricular filling
*has slower automaticity compared to the sinus node
pacemaker rates in the heart (general)
SA > AV > bundle of His/Purkinje/ventricles
speed of conduction in the heart (general)
bundle of His/Purkinje > atria > ventricles > AV node
P wave
atrial depolarization
PR interval
*time from start of atrial depolarization to start of ventricular depolarization
*normal: 120-200 msec (3-5 small boxes)
PR segment
*isoelectric
*time from END of atrial depolarization, BEFORE start of ventricular depolarization
*represents the signal as it travels through the AV node
QRS complex
*ventricular depolarization
*normal: ~ 100 msec (2-3 small boxes)
QT interval
*time from the beginning of the QRS (ventricular depolarization) to the end of the T wave (end of ventricular repolarization)
*ventricular depolarization, mechanical contraction of the ventricles, and ventricular repolarization
T wave
*ventricular repolarization
note - T-wave inversion may indicate ischemia or recent MI
J point
*junction between end of QRS complex and start of ST segment
ST segment
*isoelectric
*measured from the end of the QRS complex to the beginning of the T wave
*represents the period between ventricular depolarization and repolarization
appearance of the QRS complex
*even though it is always labeled as the “QRS”, not every QRS has to have a q wave, r wave, or s wave
*R wave is ALWAYS A POSITIVE DEFLECTION (UP)
*Q wave is a negative deflection in front of the R wave
*S wave is a negative deflection after an R wave
standard EKG speed is
25 mm/sec
standard EKG records how long?
10 seconds
vertical axis of EKG readout
*vertical axis: voltage (measured in mV)
*1 big box (composed of 5 small boxes) = 0.5 mV
*2 big boxes = 1 mV
horizontal axis of EKG readout
*horizontal axis: time (measured in msec or sec)
*1 big box = 200 ms (0.2 seconds)
*5 big boxes = 1 SECOND (1000 msec)
EKG electrodes & leads
*electrodes (stickers) detect electrical activity
*an ECG “lead” connects the electrodes to the ECG machine
*leads are specific to location; electrodes are not
*by placing them at specific, fixed locations, they enable us to understand how electricity flows through the heart
*integrating the information obtained from different leads can help up to piece together a diagnosis
direction of EKG deflections, based on direction of electrical signal (relative to the leads)
*if electrical signal is coming TOWARDS the electrode/lead, it will cause an upward or “positive” deflection
*if electrical signal is going AWAY FROM the electrode/lead, it will cause an downward or “negative” deflection
*if electrical signal is parallel (not coming toward or going away from) the lead, it will cause positive and negative deflections to be equal, or it may even be flat; this is termed “isoelectric”
mnemonics for placement of ECG leads
*WHITE to the RIGHT → white limb lead goes to the right shoulder/arm
*red to left ribs
*SMOKE OVER FIRE → black over red on the left side
*snow over grass → white over green on the right side
EKG UNIPOLAR limb leads
*unipolar leads mean these leads use only a positive electrode, and measure the flow of electricity relative to that fixed point
1. lead aVR: right shoulder
2. lead aVL: left shoulder
3. lead aVF: left leg
4. note - there is a lead to the right leg, but this is just an “electrical ground”; we don’t look at it
EKG lead: aVR
*unipolar limb lead
*placement: right shoulder
*lead color: WHITE (white to the right)
EKG lead: aVL
*unipolar limb lead
*placement: left shoulder
*lead color: BLACK (SMOKE > fire)
EKG lead: aVF
*unipolar limb lead
*placement: left leg
*lead color: RED (smoke > FIRE)
EKG BIPOLAR limb leads
*bipolar: information from 2 leads is used to create a waveform
*one electrode can be taken to be positive and the other is negative
*creates Einthoven’s Triangle
*leads I, II, and III
EKG lead: lead I
*right shoulder = negative
*left shoulder = positive
EKG lead: lead II
*right shoulder = negative
*left leg = positive
EKG lead: lead III
*left shoulder = negative
*right shoulder = positive
EKG map
*NORMAL: highest QRS between -30 degrees and +90 degrees (aVL, aVF, lead I, or lead II)
*if QRS is highest from -30 to -90, then there is LEFT AXIS DEVIATION
*if QRS if highest from +90 to +180, then there is RIGHT AXIS DEVIATION
EKG map: left axis deviation
*lead I positive; lead II negative
*if QRS is highest from -30 to -90 on EKG map
*causes may include: inferior wall MI, left anterior fascicular block, left ventricular hypertrophy
EKG map: right axis deviation
*lead I negative; aVF positive
*if QRS is highest from +90 to +180 on EKG
*causes may include: right ventricular hypertrophy, acute right heart strain, left posterior fascicular block
EKG map: severe right axis deviation
*lead I negative; aVF negative
*if QRS is highest between -90 and 180
*should never happen; leads are likely placed incorrectly
EKG precordial leads
V1: lower right sternal border
V2: lower left sternal border
V3 - V5: spaced roughly equidistant from each other between V2 and V6
V6: left midaxillary line
QRS axis based on EKG: stepwise approach
*start at lead 1: positive or negative?
-if POSITIVE: look at lead 2
-if NEGATIVE: look at lead aVF
QRS axis based on EKG: if lead 1 is positive…
*look at lead 2:
-if POSITIVE: normal
-if NEGATIVE: LEFT axis deviation
QRS axis based on EKG: if lead 1 is negative…
*look at lead aVF:
-if POSITIVE: RIGHT axis deviation
-if NEGATIVE: EXTREME right axis deviation (check your leads)
QRS axis based on EKG: lead 1 POSITIVE, lead 2 POSITIVE
NORMAL QRS axis
QRS axis based on EKG: lead 1 POSITIVE, lead 2 NEGATIVE
LEFT QRS axis deviation
QRS axis based on EKG: lead 1 NEGATIVE, lead aVF POSITIVE
RIGHT QRS axis deviation
QRS axis based on EKG: lead 1 NEGATIVE, lead aVF NEGATIVE
EXTREME RIGHT axis deviation (check your leads)
bradycardia
heart rate < 60 bpm
tachycardia
heart rate > 100 bpm
how can you calculate HR based on EKG?
- count # of QRS complexes, and multiply by 6 (b/c an EKG measures for 10 seconds)
- the 300 rule: HR = 300 divided by the number of big boxes between the QRS complexes (see image)
